The success of orthopaedic implants surgically implanted in living bone substantially depends on achieving and maintaining an enduring bond between the confronting surfaces of the implant and the host bone. Surgical procedures for preparing living bone to receive a surgically implanted orthopaedic device have been known for twenty years or more, but the ideal properties of the surface of the orthopaedic implant which confronts the host bone and processes of preparing the implant surface are the subjects of considerable disagreement.
Ultrahigh molecular weight polyethylene is widely used in the orthopaedics industry for the production of orthopaedic implants due to its relatively high wear-resistance and biocompatibility. For example, ultrahigh molecular weight polyethylene frequently is used to produce the acetabular cup of artificial hip joints. However, virgin ultrahigh molecular weight polyethylene is bio-inert, and living cells (e.g., bone cells, osteoblast-like cells, or soft tissue cells) show relatively little, if any, affinity towards such virgin ultrahigh molecular weight polyethylene. Accordingly, orthopaedic implants comprising ultrahigh molecular weight polyethylene components that must be anchored to the host bone are at risk for potential failure of the orthopaedic implant unless additional means are undertaken to ensure the establishment and maintenance of a bond between the ultrahigh molecular weight polyethylene component and the host bone.
Many techniques have been or currently are used to establish such a bond between the ultrahigh molecular weight polyethylene component and the host bone. For instance, early ultrahigh molecular weight polyethylene components, such as the acetabular cup of an artificial hip, were bonded to the host bone (i.e., the acetabulum of the pelvis) using a bone cement. Currently, some commercially available ultrahigh molecular weight polyethylene components have been provided with complex surface geometries, which surface geometries comprise ridges and/or projections, intended to provide sites for the anchoring of the ultrahigh molecular weight polyethylene component to the host bone. Alternatively, ultrahigh molecular weight polyethylene components can be anchored to the host bone using a mechanical fastener or attached to a metallic “shell” or “tray,” which shell or tray usually comprises a roughened and/or porous coating that confronts the host bone. While each of these means for effectively “bonding” the ultrahigh molecular weight polyethylene component to the host bone have enjoyed varying degrees of success, orthopaedic implants relying on such means are at risk for failure if the particular means (e.g., the ridge, protrusion, or mechanical fastener) fails.
A need therefore exists for a medical implant or medical implant part comprising ultrahigh molecular weight polyethylene that provides a substrate suitable for bone in-growth and/or soft tissue in-growth. A need also exists for a medical implant or medical implant part comprising ultrahigh molecular weight polyethylene having a structure that permits bone cement to penetrate into the medical implant or medical implant part, which can increase the strength of the bond between the medical implant or medical implant part and the host bone. The invention provides such an implant and a process for making the same. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.